In the past, if time division multiplex messages were to be transmitted from point A to point B, with the capability of having additions made to the data at intermediate points therebetween, a series of transmitters and receivers would be strung from point A through each of the intermediate points to point B, with each intermediate point having a transmitter and receiver, where the message would be demodulated to recover the required channel and timing information. Then the message would be completely remodulated, on a new carrier with the new channel added. A simple transmission code, similar to the alternate mark inversion (AMI) or bipolar code, would typically be used.
While such designs and such transmission codes have enjoyed considerable use in the past, they have several serious drawbacks. First of all, the prior art AMI transmission code does not easily allow adding new data at intermediate points without completely demodulating the message and modulating a new message with the new data added. Another problem is that the transmitter and receiver, which are associated with each intermediate point, are typically connected in series along the transmission line. This results in a reliability problem of the system. The reliability of the system is dependent upon the operation of each transmitter and receiver and if one transmitter or receiver fails the entire system is unable to function.
Consequently, there exists a need for improvements in digital audio data communication techniques which allow for data transmission from A to B, with many intermediate points for adding data therebetween, which technique does not require the complete demodulation and remodulation of the entire signal at each intermediate point. Also, a need exists for a technique which allows for a transmitter or receiver, at any intermediate point, to fail and still allow the overall data transmission system to operate.